NEW YORK, (Reuters) -- The Arctic ice shelf has melted for the fourth straight year to its smallest area in a century, driven by rising temperatures that appear linked to a buildup of greenhouse gases, U.S. scientists said Wednesday.

Scientists at NASA and the National Snow and Ice Data Center, which have monitored the ice via satellites since 1978, say the total Arctic ice in 2005 will cover the smallest area since they started measuring.

It is the least amount of Arctic ice in at least a century, according to both the satellite data and shipping data going back many more years, according to a report from the groups.

As of September 21, the Arctic sea ice area had dropped to 2.05 million square miles (5.31 million square km), the report said.

From 1978 to 2000, the sea ice area averaged 2.70 million square miles (7 million square km), the report said. It noted the melting trend had shrunk Inuit hunting grounds and endangered polar bears, seals and other wildlife.

The report warns that if melting rates continue, the summertime Arctic may be completely ice-free before the end of the century, echoing last year's findings from the Arctic Council, an eight-nation report by 250 experts.

The melting trend increasingly appeared to be caused by a buildup of greenhouse gases in the atmosphere, the scientists said.

"It's increasingly difficult to argue against the notion that at least part of what we are seeing in the Arctic, in terms of sea ice, in terms of warming temperatures ... is due to the greenhouse effect," Mark Serreze, a research scientist at NSIDC, said in an interview.

"We've put a hit on the system and we are in the midst of a grand global experiment," Serreze said about the impact of global warming and ice melting on humans and animals. "We will have to live with the outcome."

The NSIDC, part of the University of Colorado at Boulder, helps NASA analyze satellite data.

Most scientists believe greenhouse gases, including carbon dioxide that is released mainly from cars and utility smokestacks, cause global warming by trapping solar heat in the atmosphere. Many believe global warming can lead to catastrophic consequences, including raising sea levels and strengthening weather events such as hurricanes.

One Arctic variation, known as Arctic Oscillation, an atmospheric circulation pattern that can push sea ice out of the area, had become less of an influence in the region since the mid-1990s, the report said.

Inuit hunters threatened by the melting of Arctic ice plan to file a petition in December accusing the United States of violating their human rights by fueling global warming. The Bush administration has opted out of the Kyoto Treaty to reduce greenhouse gas emissions.

The Inuit number about 155,000 people in Canada, Alaska, Greenland and Russia.

Scientists say the Arctic is warming faster than the rest of the globe because water or bare earth, once uncovered, soaks up more heat than ice and snow. That process means melting can spur even warmer temperatures and more melting.